A lithium ion secondary battery, which is a kind of nonaqueous electrolyte secondary battery, is characterized by high voltage and high capacity, and therefore has been widely used. In order to achieve more effective use of the lithium ion secondary battery, various modifications have also been made to its charging method. The charging method for the lithium ion secondary battery is generally constant-current constant-voltage (CCCV) charging.
The CCCV charging is performed as shown in FIG. 6. In FIG. 6, the horizontal axis represents time and the vertical axis represents a voltage, a current, and a temperature. FIG. 6 shows changes in the voltage and the temperature when a battery is charged by controlling the current as indicated by the graph. In the early stage of charging, constant-current (CC) charging is performed. Specifically, a battery is charged with a constant current of, e.g., about 0.7C to 1C. In this case, a current value at which a fully charged battery can be discharged in one hour is defined as 1C. The CC charging is continued until the voltage of the battery is increased with charging and reaches a predetermined set voltage VC, e.g., 4.2 V. Then, the CC charging is switched to constant-voltage (CV) charging when the voltage has reached the set voltage VC, and the CV charging is performed while the charging current is reduced to maintain the set voltage VC.
In recent years, the current during the CC charging needs to be as large as possible so that the CCCV charging can be performed in a short time. The amount of charge is the integral of the charging current with respect to time. Therefore, the process of increasing the charging current is effective in reducing the charging time. However, the charging involves the generation of heat, and the amount of heat generated increases with an increase in the current. Moreover, it is known that the charge-discharge cycle characteristics of the lithium ion secondary battery are reduced when it is charged in the high temperature environment. For this reason, various quick charging methods have been proposed, which prevent the degradation of battery characteristics.
For example, Patent Document 1 proposes a charging method for a nonaqueous electrolyte secondary battery. In this method, first, the battery is charged with a constant current until a specified charging voltage. Then, the battery is charged in a stepwise manner, while the charging current is gradually reduced. Thus, the nonaqueous electrolyte secondary battery, which has a high battery voltage, can be quickly charged and maintain good cycle characteristics.
Patent Document 2 proposes a charging method for a lithium ion battery. In this method, the initial charging current is sufficiently larger than 1CA, and then the charging current is gradually reduced. Thus, the lithium ion battery can be charged in a short time and ensure the durability.
Patent Document 3 proposes a charging method for a battery. In this method, a battery voltage and a battery surface temperature are measured before the battery is charged. When the battery voltage indicates that the depth of charge is 50% or less, and the battery surface temperature is 0° C. to 60° C., constant-current charging is performed at a predetermined current value, and then constant-current constant-voltage charging is performed at a current value smaller than that current value. Thus, the cycle characteristics of the battery can be maintained while ensuring a sufficient discharge capacity in a short time.
Patent Document 4 proposes a charging system for a lithium ion battery. The charging system uses a specific variable charge-profile to apply a charging voltage and a charging current to the battery. Thus, the charging time of the lithium ion battery can be further reduced while suppressing or eliminating the influence of quick charging on the cycle life.
On the other hand, the relationship between the state of charge of a lithium ion secondary battery and the diffusion coefficient of lithium ions in a negative electrode has become clear recently (see Non-Patent Document 1). In the present specification, the diffusion coefficient of lithium ions in a negative electrode means a physical constant representing the ease of movement of lithium ions in a negative electrode, and is determined by the type of negative electrode active material. FIG. 5 of Non-Patent Document 1 shows the relationship between the state of charge and the diffusion coefficient of lithium ions when graphite is used as a negative electrode active material. It is evident from FIG. 5 of Non-Patent Document 1 that the diffusion coefficient of lithium ions varies with the state of charge for a graphite negative electrode.
In general, the movement of lithium ions during the charging of the lithium ion secondary battery is limited on the negative electrode side. Therefore, even if the charging current is increased to shorten the charging time, the charging efficiency is reduced when the charging current exceeds the ability of the negative electrode to accept lithium ions. This increases the amount of heat generated by the battery and leads to the degradation of the battery characteristics. In this case, the ability of the negative electrode to accept lithium ions is considered to depend on the diffusion coefficient of lithium ions in the negative electrode. Thus, if a charging method can be developed by taking into account the diffusion coefficient of lithium ions in the negative electrode, the charging method would be efficient and prevent the degradation of the battery characteristics.